Field of the Invention
The present invention relates to a projection image generation apparatus, method, and program, and more specifically to a projection image generation apparatus, method, and program for generating, from three-dimensional image data representing an interior of a subject, a virtual endoscopic image, which is a pseudo three-dimensional image, for visualizing an interior of a lumen of the subject.
Description of the Related Art
Recently, with the advancement of imaging devices (modalities), resolution of image data obtained by imaging devices has been improved and detailed analysis of a subject based on image data has become possible. For example, multi detector-row computed tomography can capture a plurality of tomographic images at a time with a thin slice thickness. The reduced slice thickness may improve the resolution of three-dimensional image, formed of a plurality of tomographic images stacked on top of each other, in a body axis direction, whereby more detailed three-dimensional image can be obtained. By displaying and analyzing such three-dimensional data, it is possible to find a lesion or the like which, heretofore, has been difficult to find.
Three-dimensional data is not suitable for direct observation by the human eye. Therefore, three-dimensional data is generally displayed after being transformed into a pseudo three-dimensional image from an arbitrary viewpoint. With the improvement of three-dimensional data resolution, a high quality three-dimensional image can be generated and, as a consequence, a virtual endoscopic image display method has been proposed as an applied technology of three-dimensional image rendering. The virtual endoscopic display method is a method in which a viewpoint is set inside of a lumen and a perspective projection image is generated and displayed based on the viewpoint. The virtual endoscopic display may provide images which seem as if they were captured by the camera of an endoscope while being moved inside of a body by sequentially changing the viewpoint by the user.
In the virtual endoscopic display, inner walls of organs having a luminal structure, such as large intestines, bronchi, blood vessels, digestive organs, and the like are displayed (visualized). In the large intestine examination, in particular, the virtual endoscopic display is gaining popularity instead of actual endoscopic examination. In the examination of bronchial tubes, understanding of the branching structure of bronchial tube is often practiced using the virtual endoscopic display or cross-section display of an image as a preliminary simulation prior to a bronchoscopic examination or navigation during the examination.
For virtual endoscopic image generation methods, a surface rendering method and a volume rendering method are known. The surface rendering method realizes the visualization by extracting a lumen and building a surface model. The volume rendering method realizes the visualization by allocating opacity and chromaticity to three-dimensional data and performing ray casting. In either display method, a virtual endoscopic image can be obtained by setting a viewpoint inside of a lumen and performing visualization processing.
In the visualization by the volume rendering method, it is necessary to allocate appropriate opacity and chromaticity to three-dimensional data in order to determine a desired structure to be examined. In the virtual endoscopic display, a virtual endoscopic image is generated by allocating transparent opacity to a portion of the data corresponding to an inner cavity where a viewpoint is set and opaque opacity to a portion of the data corresponding to the inner wall. It is often the case that the opacity and chromaticity are allocated to pixel values (voxel values) constituting three-dimensional data, which are known as opacity curve setting and color mapping respectively. In most cases, these are parameters manually set by the user subjectively, but a method of automatically setting the opacity by combining a pixel value distribution, values of first and quadratic differential with the parameters is also proposed.
Japanese Unexamined Patent Publication No. 2004-283373 (paragraphs 0127 to 0131) is a document in which a method that selectively uses a plurality of opacity curves is described. In Japanese Unexamined Patent Publication No. 2004-283373, two opacity curves are selectively used. The first opacity curve is used to transparently display the inside of a blood vessel, while the second opacity curve is used to transparently display the outside of a blood vessel. The first opacity curve is applied when a viewpoint is outside of the blood vessel, while the second opacity curve is applied when a viewpoint is inside of the blood vessel. Japanese Unexamined Patent Publication No. 2004-283373 describes that a weighted average of pixel values adjacent to a viewpoint is taken, and one of the two opacity curves having lower opacity in terms of the average pixel value is used.
Here, if the luminal organ of the observation target is an organ of large structure, such as a large intestine, pixel values of an inner cavity portion as the air space are substantially constant and, therefore, an inner wall portion can be observed with the same opacity setting regardless of where in the lumen the viewpoint is located. But, for bronchial tubes and blood vessels, they gradually taper, as the luminal structure, toward the end, and greatly differ in the pixel value between proximal and distal end portions thereof. Consequently, if a virtual endoscopic image is generated using the same opacity setting between the proximal and distal end portions, the inner wall at the distal end portion can not be displayed although the inner wall at the proximal end portion can be displayed.
In Japanese Unexamined Patent Publication No. 2004-283373, two opacity curves are selectively used according to the situation. In Japanese Unexamined Patent Publication No. 2004-283373, however, two opacity curves are selectively used according only to whether or not the viewpoint is inside of a blood vessel. Japanese Unexamined Patent Publication No. 2004-283373 describes, in paragraph 0131, that it is necessary to finely adjust the opacity curves depending on the observation area of a blood vessel, but does not solve the aforementioned problem at all.
It is an object of the present invention to provide a projection image generation method and apparatus capable of solving the problem of the conventional technology described above and visualizing an interior of a lumen even when the position of the viewpoint is changed in virtual endoscopic image generation. It is a further object of the present invention to provide a computer readable recording medium on which is recorded a program for causing a computer to perform the projection image generation method.